1. Field of the Invention
The present invention relates to a motor unit that turns a switch on and off to perform cyclic motor energization and de-energization so that a single-phase or polyphase motor is driven with the number of revolutions that corresponds to the relevant percent energization. The invention also relates to an associated motor drive unit, a method of its control, and a fan unit.
2. Description of the Related Art
FIG. 59 is a circuit diagram illustrating a prior art motor drive unit as typically described in Japanese Patent Application No. 62-239895. Referring to FIG. 59, numeral 1 designates a commercial AC power supply of 100 V, 2 is a fan motor, 3 is a power transformer, 4 is a diode bridge, 5 is an electrolytic capacitor, and 6 is a three-terminal regulator. The diode bride 4, electrolytic capacitor 5 and three-terminal regulator 6 combine to form a constant-voltage power supply circuit.
Numerals 7 and 8 designate diodes, 9, 10 and 12 are resistors, and 11 is a transistor. Diodes 7 and 8, resistors 9, 10 and 12, and transistor 11 combine to form a zero-crossing detector circuit.
When the AC 100 V power supply 1 is zero-crossing, the potential difference across the secondary winding of the power transformer 3 is almost equal to zero, so the potential at either terminal of the secondary winding is lower than the GND level at the output of diode bridge 4 (i.e., the negative terminal of electrolytic capacitor 5). The GND level is normally higher than the lower of the potentials at the two terminals of the secondary winding of power transformer 3 and the difference is equivalent to the forward voltage at the diode in the diode bridge 4. Therefore, both diodes 7 and 8 will be reverse-biased and no base current can flow through transistor 11. Hence, transistor 11 is off and pull-up resistor 12 allows the collector potential of transistor 11 to be equal to the supply voltage of microcomputer 13, thereby developing "H" (high) level.
When the AC 100 V power supply 1 is not zero-crossing, the potential at either terminal of the secondary winding of power transformer 3 becomes higher than the potential at the other terminal and a base current will flow through diode 7 or 8 and resistor 9, causing transistor 11 to turn on; as a result, the collector potential of this transistor is on (at GND level), thereby developing "L" (low) level.
Thus, the collector potential of transistor 11 can be used as an indicator for detecting whether the AC 100 V power supply 1 is zero-crossing or not.
Shown by 13 in FIG. 59 is a microcomputer as an electronic circuit for controlling the percent energization of fan motor 2. The microcomputer has a power supply terminal P1, which is given an output (Vcc) from the constant-voltage power supply circuit composed of diode bridge 4, electrolytic capacitor 5 and three-terminal regulator 6. The microcomputer 13 also has a ground terminal P2 connected to GND (0 V), an input terminal P3 which is connected to the collector of transistor 11 (i.e., the output terminal of the zero-crossing detector circuit composed of diodes 7 and 8, resistors 9, 10 and 12, and transistor 11), and an output terminal P4 which is connected via resistor 18 to resistor 19 and the base of transistor 20. Shown by 22a is the light-emitting side of a photo-triac coupler and couples optically to the light-receiving side 22b of the coupler, which is an integral combination of 22a and 22b. The microcomputer 13 has an external input terminal P5 at which a command is entered to signal the percent energization of the electronic control circuit composed of the microcomputer 13. In the case under consideration, terminal P5 is controlled with another microcomputer.
If the output terminal P4 of microcomputer 13 becomes "H", transistor 20 will turn on and a trigger current will flow to the light-emitting side 22a of the photo-triac coupler, causing the light-receiving side 22b to turn on. As a result, a gate trigger circuit for triac 27 that is composed of resistors 23 and 24 is closed and the triac 27 which composes a switching circuit together with the gate trigger circuit turns on to energize fan motor 2.
When the output terminal P4 of microcomputer 13 is "L", transistor 20 is off and no trigger current will flow to the light-emitting side 22a of the photo-triac coupler; hence, the light-receiving side 22b of the photo-triac coupler will turn off and the gate trigger circuit will open, whereupon the triac 27 turns off. Hence, fan motor 2 is not energized. Shown by 25 and 26 are a resistor and a capacitor, respectively, which combine to compose a snubber circuit for the triac 27.
We now describe specific means for controlling a fan motor. As described in a prior art publication, "HANDBOOK 0F ELECTRIC ENGINEERING", 1988 Edition, Section 16, pages 724 to 725, phase control is a technique widely adopted as means for achieving variable speed control of capacitor motors. An energization waveform and other waveforms related to phase control are shown by time charts as Prior Art Case 1 in FIG. 60. Referring to FIG. 60, numeral 28 indicates the waveform from the AC 100 V power supply 1; numeral 29 indicates the waveform of a zero-crossing signal to input terminal P3 of microcomputer 13; numeral 30 indicates the waveform to output terminal P4 of microcomputer 13; and numeral 31 indicates the waveform of a current being applied to energize fan motor 2 (which may be called the "energization waveform").
If the time from zero-crossing to the turning-on of triac 27 is written as .beta., one can control not only the power to be supplied to the motor but also the number of its revolutions by adjusting the value of .beta.. For example, full energization is achieved if .beta. is zero but there will be not energization at all if .beta. is equal to one half the power source period. FIG. 60 refers to the case where .beta. is a quarter of the power source period and the power to be supplied to the motor is adjusted to be about one half the value from the power supply 1.
FIG. 61 shows various waveforms for illustrating Prior Art Case 2 which is described in Japanese Patent Application No. 62-239895, and FIG. 62 is a memory map showing patterns for energization of fan motor 2.
In order to realize the most appropriate percent energization for attaining the number of revolutions of fan motor 2 that is determined by control factors such as time and the temperature of the heat exchanger in a heater, the microcomputer 13 will energize fan motor 2 in accordance with patterns typically described in the FIG. 62 memory map. The memory map has patterns that consist of a total of 24 cycles for each value of energization from the AC 100 V power supply 1, and every shift from one on-off pattern to the next is made after 6 cycles (24 cycles divided by 4).
Consider, for example, the case of 83.3% energization. Fan motor 2 is energized according to the following patterns in the memory map;
5 cycles ON and 1 cycle OFF PA1 5 cycles ON and 1 cycle OFF PA1 5 cycles ON and 1 cycle OFF PA1 5 cycles ON and 1 cycle OFF PA1 2 cycles ON and 4 cycles OFF PA1 1 cycle ON and 5 cycles OFF PA1 1 cycle ON and 5 cycles OFF PA1 1 cycle ON and 5 cycles OFF
Since the total number of energization cycles is 20 (=5+5+5 +5) out of the 24 cycles, the percent energization is 20/24 =0.833.
If 20.8% energization is necessary, fan motor 2 is energized according to the following patterns in the memory map;
Since the total number of energization cycles is 5 (=2+1+1+1) out of the 24 cycles, the percent energization is 5/24 =0.208.
As is also clear from FIG. 62, energization or de-energization cycles occur in a minimum unit of one.
Problems to be Solved by the Invention:
Being constructed in the manner described above, the prior art motor unit and motor drive unit have suffered from the noise problem during the driving of the motor. The present inventors conducted extensive studies on the development of noise and have found the following possible causes.
First, in Prior Art Case 1, switching at a frequency of 2f causes torque pulsations in the capacitor motor at 2f and at integral multiples of 2f, thereby developing magnetic sound at 2f and at integral multiples of 2f. Additionally, the capacitor motor normally turns on at times that are offset from zero-crossing points of the supply voltage and, hence, at the moment the motor turns on, a current will flow so abruptly that a great vibrational force and a large magnetic sound will occur.
Secondary, in Prior Art Case 2, 5 or more ON cycles are employed in the range of "high values of percent energization" as shown in FIG. 62 and, in those periods where 5 or more 0N cycles occurs, the capacitor motor will experience 2f torque pulsations and produce 2f magnetic sound. Additionally, the basic number of cycles for the shift from one ON-OFF pattern to the next is 6 and, therefore, if the supply frequency (f) is 60 Hz, ON and OFF cycles will be repeated at every 10 Hz. With such low repetition frequency, ON and OFF cycles may occasionally be discernible by the auditory sense and will produce intermittent sounds that jar on the ear as noise.
Additionally, if mechanical resonance occurs in a single-phase or polyphase motor or when the main and auxiliary windings on a single-phase motor are simultaneously subjected to control over energization and de-energization, a magnet center displacement and other defects will cause the shaft of the fan motor to vibrate greatly in the axial direction, producing abnormal vibrations as exemplified by the occurrence of a "tapping" sound.
The conventional method of controlling the number of revolutions of a motor has had another problem in that noise and vibrations occur during the drive of the motor. A cause of the occurrence of noise and vibrations is that since motor energization is subjected to cyclic on-off control in accordance with predetermined patterns in a memory map, the repetition is occasionally discernible as a "tapping" sound by the auditory sense and the base frequency due to the repetition of ON and OFF cycles causes noise and vibrations.
Additionally, the fan being driven by the motor produces continual "swishes" that also jar on the ear.